Figure 2. Four levels of hierarchical organization.

All views are from outside, but only the floor regions of MCP monomers are shown. We use three color schemes to highlight different organizational levels, one for panels (a) and (b) as in Figure 1d, a second for (c), and a third for (d) and Fig. S4. Panels (a) to (c) contain all of the same monomers. Panel (d) is an enlarged version of the center of panel (c). (a) Within a capsomer, each MCP subunit interacts with its neighbors. MCP subunits within each capsomer are marked by the same color (orange, green or blue), and the orange capsomer is enclosed by a hexagon. (b) Binding of neighboring capsomers (bright and pale orange MCP subunits, bright and pale green, bright and pale blue) in groups of two by interactions between MCP dimerization domains (marked by rectangles – see inset) across local two-fold axes (‘2’), binding the orange capsomer with the green, the green with the blue, and the blue with the orange). Also, triplex heterotrimers bind groups of three (orange, green and blue) capsomers at the local 3-fold axis marked by ‘3’. (c) The magenta belt, surrounding the outlined capsomer, is created by a ring of six Johnson folds from six (magenta) MCP subunits – each subunit a member of a different capsomer – joined by triplex heterotrimers. We also show yellow and red belts, the latter with numbered MCP subunits. (d) Formation of non-covalent chainmail by concatenated belts. Each of the three belts is joined by one subunit of the triplex heterotrimer (not shown) centered at the position marked by ‘3’, each joint denoted by a differently colored curly brace to correspond with its (pink, cyan or green) triplex monomer, consistent with the color scheme in Figure 6. See also Fig. S4. (e) Belts (e.g., red crossing over magenta, over yellow, under magenta, and under yellow) in the three-fold symmetric pattern of the RRV capsid do not separate if one belt breaks, thus forming chainmail. By contrast, Borromean rings, with a strictly alternating pattern (e.g., red crossing under magenta, over yellow, under magenta again, over yellow again), do separate if any one ring breaks. Nonetheless, at local three-fold axes, the crossing patterns are the same (inset). See also Movie S3.